This invention relates to a tone signal generation device adapted for use in an electronic musical instrument and other apparatus having a tone generation function and, more particularly, to a tone signal generation device capable of generating a tone signal whose spectrum components change with the lapse of time by successively generating different tone waveshapes as well as capable of generating a tone signal containing a non-harmonic component.
Japanese Preliminary Patent Publication No. 95790/1983 discloses a tone signal generation device capable of generating a tone signal whose spectrum components change with the lapse of time by successively reading out different tone waveshapes stored in a waveshape memory. In this device, switching of tone waveshapes to be read out from the memory is effected each time the same tone waveshape has been repeatedly read out for a given number of periods. Besides, for smooth transition from a preceding tone waveshape to a next tone waveshape at the time of switching, an interpolation between corresponding sample points of the two waveshapes is performed and this interpolation is carried out for the given number of periods during which the same tone waveshape is repeatedly read out.
In the above described prior art device, the interval between switchings of tone waveshapes is fixed to a predetermined number of periods and, accordingly, the interval of switching varies depending upon the frequency of a tone to be generated and therefore time required for the interpolation varies. This gives rise to the problem that the higher the frequency of a tone, the faster the tone waveshape switches with a result that the timewise change effect of the spectrum components become unequal depending upon the tone pitch. Further, the higher the frequency of a tone, the faster is performed the interpolation at the switching between the waveshapes so that the effect of the smooth transition between the different waveshapes is reduced.
It is, therefore, an object of the invention to provide a tone signal generation device which has eliminated the above described disadvantages of the prior art device by controlling the switching between the tone waveshapes without being affected by the frequency of a tone to be generated.
The above described prior art device discloses also an art of interpolation according to which weighting is carried out with respect to both a preceding waveshape and a following waveshape at the time of switching between the waveshapes so as to realize smooth transition from the former to the latter. Since in the disclosed method of interpolation, difference between the preceding tone waveshape and the following tone waveshape is computed for each corresponding sample point and this difference is multiplied with a weighting coefficient and thereafter the product is added to sample point amplitude data of the preceding tone waveshape, gain of a tone waveshape signal finally obtained is always "1" no matter what value the weighting coefficient may be with resulting lack in variety in the interpolation characteristics. Assuming that the amplitude level of a preceding waveshape is represented by A, that of a following waveshape by B, a weighting coefficient by X, the level of a tone waveshape signal obtained is A+X(B-A)=A(1-X)+BX and gain is always "1". Besides, in a case where a function of a weighting coefficient (interpolation function) is determined as desired, the above described method of interpolation produces deviation in the interpolation characteristics so that a smooth interpolation cannot be expected. For instance, in a case where the weighting coefficient changes exponentially with respect to time, the level of the preceding tone waveshape tends to remain in a high value and the level of the following waveshape increases abruptly immediately before the end of the interpolation with a result that a smooth interpolation cannot be expected. Accordingly, it is only in the interpolation in a linear section that an impartial and smooth interpolation can be realized.
It is therefore a second object of the invention to provide a tone signal generation device capable of setting desired interpolation characteristics by freely setting a function concerning weighting and moreover capable of eliminating the deviation in the interpolation characteristics and realizing smooth transition of a tone waveshape.
Tones produced by acoustic musical instruments, particularly string-striking musical instruments such as piano and harpsichord, contain components which are not in an exact harmonic relationship of the notes of these tones (i.e., nonharmonic components). Since in the known tone signal generation system in which tone waveshapes stored in a waveshape memory are simply read out repeatedly can produce only harmonics of integer multiples, such known system cannot produce a tone signal containing a nonharmonic component. On the other hand, an electronic musical instrument of a type in which individual harmonic components are separately calculated and synthesized together, synthesis of a tone signal containing nonharmonic components is possible as is disclosed in the specification of U.S. Pat. No. 3,888,153. More specifically, a partial tone signal of a nonharmonic component is generated by causing the frequency of each individually generated harmonic component to deviate slightly from an integer multiple of the fundamental frequency as required and then partial tone signals are synthesized with the nonharmonic partial tone signal to provide a tone signal containing a nonharmonic component.
This prior art device however has the disadvantage that it requires a large-scale hardware because it necessitates a construction in which partial tone signals corresponding to the fundamental wave and respective harmonics must be produced individually and separately and relative amplitudes of these partial tone signals must be individually controlled before synthesizing these signals.
It is therefore a third object of the invention to provide a tone signal generation device capable of readily producing a tone signal containing a nonharmonic component with a relatively simple construction.